how can i play the multiplayer servers please(asking for cd key)

hey what up. so just got back into this game after like 5 years and now steam wont give me a cd key to play and when i make my account in the game menu its asking for a cd key and i dont want to play single player i want to blow up some building in multiplayer please man. can you help me get the online working my friend i bought this game on xbox 360 back in the day and now pc but i cant enjoy it